1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method.
2. Description of the Related Art
Generally, a so-called tandem type color image forming apparatus is known as an electrophotographic color image forming apparatus such as a color copying machine, a color printer, and a color facsimile. In the tandem type color image forming apparatus having black (K), yellow (Y), magenta (M), and cyan (C) image forming units, toner images formed by the image forming units are sequentially transferred onto an intermediate transfer member, and the toner images on the intermediate transfer member are transferred onto a recording medium, thereby forming the color image on the recording medium.
In the tandem type color image forming apparatus, when rough paper such as embossed paper having low surface smoothness is used as the recording medium, a transfer potential at a recessed portion in a surface of the recording medium is lower than that of a projected portion. Therefore, the image forming unit for forming a transparent toner image is disposed on the upper-most stream side in a process direction, the toner image formed by the transparent toner is transferred onto the intermediate transfer member, and the black (K), yellow (Y), magenta (M), and cyan (C) toner images are transferred onto the transparent toner image while superposed on one another. This enables a transfer property to be improved when the toner images on the intermediate transfer member are transferred onto the recording medium.
In the case where plain paper having the high surface smoothness is used as the recording medium in the image forming apparatus having the above configuration, namely, even in the case where the transfer of the transparent toner image onto the intermediate transfer member is not required, when a transparent-toner image forming unit (photosensitive drum) is not retracted from the intermediate transfer member but is always brought into contact with the intermediate transfer member, the transparent-toner photosensitive drum is abraded to shorten a lifetime of the image forming unit.
However, in the configuration in which the image forming unit is retracted, a gear mechanism for moving the image forming unit is easily damaged.
Additionally, the color shift is increased among each of the color toner image transferred onto the intermediate transfer member due to eccentricity of the image carrier or gear.